So, today was a weird day...
Finally got a warm day so I decided to try some more troubleshooting. Ruck didn't want to start initially; took a bit of starting fluid to get it going and it had to warm up a bit before it'd idle properly. Was a bit concerning as up to this point I'd never had a problem starting or idling the ruck, even in cold weather.
I took the intake off to visually confirm that the carb slide was moving. It is, though it seems to be a bit jittery (vacuum leak? or maybe this is normal...). Burped the radiator as someone had suggested just to be sure, and took the ruck around the block.
It ran perfectly. No bogging at all.
Examined the snorkel and it did have a tiny split in it near the filter end. Mounted the filter directly to the carb (clears the rear wheel by about an inch) and took it out for a proper ride and a bit of shopping. Not a single problem all day.
Either my bike is magic and mends itself, or it's one temperamental bitch. It leaves me more confused every time I look at it.
Let it cool back down while I did other stuff, and went out to cold-start it again. Fired up second try, but needed a bit of throttle to idle until it had warmed up a bit. Perhaps the cold-start enrichment thing is borked?